H04B10/291—Repeaters in which processing or amplification is carried out without conversion of the main signal from optical form

H—ELECTRICITY

H04—ELECTRIC COMMUNICATION TECHNIQUE

H04J—MULTIPLEX COMMUNICATION

H04J14/00—Optical multiplex systems

H04J14/02—Wavelength-division multiplex systems

H04J14/0221—Power control, e.g. to keep the total optical power constant

Abstract

Optical systems of the present invention generally include at least one optical amplifier having a first pump source configured to supply power to said amplifier via a first pumping paths. A second pumping path is provided to supply power to the amplifier from a second pump source configured to replace power from the first pump source. The amplifier allows the replacement and repair of a pump source during operation by providing in-service hot swap capability, which increases the overall availability of the amplifier. The pump source can also be changed during operation to allow reconfiguration of the optical system.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 09/310,324, filed May 12, 1999, now U.S. Pat. No. 6,567,207.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

The present invention is directed generally to optical transmission systems. More particularly, the invention is directed toward optical transmission systems including optical amplifiers configured to provide for in-service pump maintenance, replacement, and upgrades.

In response to this demand, communications service providers have turned to optical communication systems, which have the capability to provide substantially larger information bandwidth transmission capacities than traditional electrical communication systems. Information can be transported through optical systems in audio, video, data, or other signal formats analogous to electrical systems. Likewise, optical systems can be used in telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems.

Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one information signal using a single wavelength in separate waveguides, i.e. fiber optic strand. The transmission capacity of optical systems was increased by time division multiplexing (TDM) multiple low bit rate, information signals into a higher bit rate signals that can be transported on a single optical wavelength. The low bit rate information carried by the TDM optical signal can then be separated from the higher bit rate signal following transmission through the optical system.

The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.

In WDM transmission systems, pluralities of distinct TDM or SDM information signals are carried using electromagnetic waves having different wavelengths in the optical spectrum, i.e., far-UV to far-infrared. The pluralities of information carrying wavelengths are combined into a multiple wavelength WDM optical signal that is transmitted in a single waveguide. In this manner, WDM systems can increase the transmission capacity of existing SDM/TDM systems by a factor equal to the number of wavelengths used in the WDM system.

Optical WDM systems were not initially deployed, in part, because of the high cost of electrical signal regeneration/amplification equipment required to compensate for signal attenuation for each optical wavelength throughout the system. The development of the erbium doped fiber optical amplifier (EDFA) provided a cost effective means to optically regenerate attenuated optical signal wavelengths in the 1550 nm range. In addition, the 1550 nm signal wavelength range coincides with a low loss transmission window in silica based optical fibers, which allowed EDFAs to be spaced further apart than conventional electrical regenerators.

The use of EDFAs essentially eliminated the need for, and the associated costs of, electrical signal regeneration/amplification equipment to compensate for signal attenuation in many systems. The dramatic reduction in the number of electrical regenerators in the systems, made the installation of WDM systems in the remaining electrical regenerators a cost effective means to increase optical network capacity.

The increased capacity of information transmitted in WDM systems has made system downtime increasingly more expensive in terms of lost capacity and revenue. Furthermore, the amount of excess system capacity must be increased accordingly to ensure a continued quality of service to customers in the event of a shutdown in part or all of the optical system.

While EDFAs have contributed greatly to the development of WDM systems, EDFAs are a source of failure that can result in the shutdown of an optical link in the system. More specifically, lasers and other optical sources, used in pump sources to supply optical energy to an erbium doped fiber in an EDFA, have proven to be a common point of failure in optically pumped amplifiers.

A pump source failure generally causes an amplifier and optical link shutdown resulting in lost revenues, degraded service quality, and unscheduled labor costs. Because of the high cost of amplifier shutdowns, pump sources generally include redundant optical sources to improve the reliability of the amplifiers.

The need for and use of redundant optical sources doesreduce the power requirements and cost, as well as increase the reliability, of the optical sources. In various EDFA designs, multiple optical sources are combined and used to pump multiple fiber amplifiers to provide redundancy and to more cost effectively deploy the optical sources. For example, see U.S. Pat. Nos. 4,699,452, 5,039,199, and 5,173,957.

The use of redundant optical sources does not improve the reliability of the optical sources, but only the reliability of the optical amplifier. For example, the failure of an optical source in amplifiers having redundant optical sources will generally not cause an unplanned amplifier shutdown, but will usually require a planned maintenance shutdown to repair or replace the failed optical source. A planned maintenance shutdown allows communications traffic to be rerouted to provide continued service and work scheduled, which greatly reduces the cost of the maintenance. Nonetheless, traffic must still be rerouted and the link taken out of service to repair the failed or degraded pump sources resulting in lost revenue and increased costs.

The provisioning of optical systems and the traffic planning costs associated with rerouting of traffic imposes a substantial financial burden on communications service providers. Rerouting of traffic is a particular concern in optical systems that do not have a highly connected network or significant excess capacity to reroute traffic.

Given the trend of continually increasing communication traffic is not expected to diminish, the need for more reliable, higher capacity optical systems will continue to grow. Thus, it is imperative that optical systems be developed including optical amplifiers with increased reliability to allow the optical systems to be operated in an efficient, cost-effective manner.

BRIEF SUMMARY OF THE INVENTION

The apparatuses and methods of the present invention address the above need for more reliable optical systems and optical amplifiers. Optical systems of the present invention includes at least one optical amplifier having a first pump source configured to supply power to the amplifier via a first pumping path. The first pump source can include one or more optical sources, such as DFB or other lasers, that can provide optical energy in narrow or broad wavelength bands to an amplifying fiber.

A second pumping path is also provided to supply power to the amplifier from a second pump source configured to replace power from the first pump source. The first and second pump sources can be operated to replace power from one pump source with power from the other pump source. A controlled replacement of the power allows amplifier performance characteristics, such as the gain and gain profile, over a signal wavelength range to be maintained during pump source maintenance. In various embodiments, additional pumping paths and pump sources can be provided in the optical amplifier and analogously operated.

The optical amplifier can include various combinations of doped fiber amplifiers, such as EDFAs, and Raman fiber amplifiers in one or more amplifier stages and in concentrated, or lumped, and distributed form. In various embodiments, the first and second pumping path provide optical energy, “pump power” travelling in the same direction in the amplifier. In other embodiments, the pump power in the first and second pumping paths travels in opposite directions in the amplifying fiber. Similarly, the pump power can co-propagate and/or counter-propagate with communication signals being unidirectionally or bidirectionally transmitted in the fiber.

Pump power from the optical sources can be combined using one or more combiners as known in the art and in one or more stages as may be appropriate. In various embodiments, standard and WDM couplers, circulators, dichroic devices, prisms, gratings, and other combiners can be used with or without wavelength selective reflective and transmissive elements, such as Bragg gratings and Fabry-Perot or other filters.

In operation, the second pump source can be removably installed to provide pump power to the optical amplifier via the second pumping path. Once in place, the second pump source can be used to replace the pump power supplied by the first pump source to the amplifier and maintain control over the amplifier performance characteristics. When the power has been reduced to a safe level, the first pump source can be replaced or repaired as required. The replacement of pump power can be performed manually be service personnel, but more desirably would be computer controlled and implemented either locally or remotely depending upon the optical system embodiment.

The first pump source can be replaced with a replacement pump source, which will typically be a replacement first pump source, but can also be an upgraded or other pump source. The pump power provided by the second pump source can be replaced by the replacement pump source and the second pump source can be removed. Alternatively, the amplifier can be designed to operate with the second pump source during normal operation.

Accordingly, the present invention addresses the aforementioned need for more reliable optical systems and amplifiers. The improvement in reliability also provides increasingly flexible and upgradable optical amplifiers with higher availability for use in future optical systems. These advantages and others will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings for the purpose of illustrating present embodiments only and not for purposes of limiting the same, wherein like members bear like reference numerals and:

FIGS. 1 and 2 show optical system embodiments;

FIGS. 3(a-l) show optical amplifier embodiments; and,

FIG. 4 shows an exemplary pump source embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Optical systems 10 of the present invention include an optical amplifier 12 disposed along an optical transmission fiber 14 to optically amplify optical signal passing between optical processing nodes 16. The optical system 10 will generally include one or more transmitters 18 configured to transmit information via one or more information carrying signal wavelength, or channels, (“signal wavelengths”) λi to one or more optical receivers 20. The optical system 10 can be configured in serially connected point to point links (FIG. 1) or in multi-dimensional networks controlled by a network management system 22 (FIG. 2).

The optical processing nodes 16 may also include other optical components, such as one or more add/drop devices and optical switches, routers, or cross-connects interconnecting the transmitters 18 and receivers 20. For example, broadcast and/or wavelength reusable, add/drop devices, and optical and electrical/digital cross connect switches and touters can be configured via the network management system 22 in various topologies, i.e., rings, mesh, etc. to provide a desired network connectivity.

The transmitters 18 transmit information in one or more signal wavelengths λi, which can be combined using an optical combiner 24 into a WDM optical signal and transmitted through the fiber 14. The transmitters 18 can include directly or externally modulated optical carrier sources or optical upconverters as known in the art. Likewise, optical distributors 26 can be provided to distribute the single or multiple wavelength optical signals to the receivers 20, which can include both direct and coherent receivers. For example, N transmitters 18 can be used to transmit M different signal wavelengths to J different receivers 20. In various embodiments, one or more of the transmitters 18 and receivers 20 can be wavelength tunable to provide wavelength allocation flexibility in the optical system 10.

The optical combiners 24 and distributors 26 can include wavelength selective and non-selective (“passive”) devices, as well as polarization sensitive devices. Standard or WDM couplers/splitters 28, circulators 30, dichroic devices 32, prisms, gratings, etc., which can be used alone or in combination with various tunable or fixed transmissive or reflective filters, such as Bragg gratings 34, Fabry-Perot devices, etc. in various configurations of the optical combiners 24 and distributors 26. Furthermore, the combiners 24 and distributors 26 can include one or more stages incorporating various devices to multiplex and demultiplex signal wavelengths λi in WDM optical systems 10.

FIGS. 3(a-l) show various embodiments of the optical amplifier 12 of the present invention. The amplifier 12 generally includes one or more amplifying media 40 supplied with power from a pump source 42 via a pumping path 44. It will be appreciated that the amplifier 12 can be a single stage or include multiple stages having the same or different amplifier designs in each stage. Also, the pump source 42 can be located proximate or remotely from the amplifying medium 40. In addition, the amplifier stages can be isolated and various signal processing, such as supervisory/service channel and signal wavelength add/drop, dispersion compensation, etc. can be performed between or proximate the stages.

Generally, the amplifying medium 40 will be a doped amplifying fiber and/or an amplifying fiber suitable for producing Raman gain in the signal wavelengths λi. One or more dopants can be used in the doped amplifying fiber 40 can include erbium, other rare earth elements such as Yb and Nd, as well as other dopants. For convenience, the amplifying medium will be generally described in terms of an amplifying fiber, also denoted as 40, but other amplifying medium, i.e., semiconductor, etc., 40 may be substituted by the skilled artisan. The doped and Raman amplifying fiber 40 can be provided as lumped or concentrated amplifiers at discrete amplifier sites and/or distributed in the transmission fiber 14.

The amplifying fiber can have the same or different characteristics than the transmission fiber 14. For example, dispersion compensating fiber, dispersion shifted fibers, standard single mode fiber and other fiber types can be intermixed as or with the transmission fiber 14 depending upon the system configuration. Thus, the amplifying fiber 40 can serve multiple purpose in the optical system, such as performing dispersion compensation in addition to amplification of the signal wavelength.

As shown in FIGS. 3(a-l) the amplifier 12 generally includes a first pump source 421 that provides optical energy, or pump power, to the amplifying medium/fiber 40 via a first pumping path 441. A second pumping path 442 to the amplifying medium/fiber 40 is also provided in the amplifier 12, which can be used or unused during normal amplifier operation. A second pump source 422 is provided to supply power to the amplifying fiber 40 via the second pumping path 442 to replace power supplied by the first pump source 421.

In FIG. 3(a), the first and second pump sources 421 and 422 supply power via pump wavelengths λp1 and λp2 to the amplifying fiber 40 in opposite directions. The signal wavelengths λi can be traveling uni-directionally or bi-directionally to and through the amplifying fiber 40, as previously described.

The pump wavelengths λp1 and λp2 supplied by the pump sources 42 can be the same or different wavelengths within the same wavelength range or different wavelength ranges. For example, pump wavelengths in the 1480 nm, and 980 nm wavelength ranges, as well as other less common ranges can be used to pump erbium doped fiber. Likewise, the pump wavelengths can be at the same or different wavelengths within wavelength ranges that will stimulate Raman amplification of signal wavelengths. For example, pump wavelengths in the 1420-1520 nm range can be used to produce Raman gain in signal wavelengths in the 1520-1620 nm range. Likewise, other pump and signal wavelengths can also be used depending upon the optical system 10 and the transmission fiber 14.

It should be noted that when different pump wavelengths are supplied, the power required to maintain or otherwise control the amplifier performance characteristics will generally be different. Likewise, the power requirements may also vary depending upon the direction traveled by the pump power relative to the signal wavelengths, even if the pump wavelengths are the same from the two pump sources.

The choice of pump wavelengths can affect the choice of combiners 24 used to deliver the pump power to the amplifying fiber 40. For example, when couplers 28 are used in the configuration of FIG. 3(a) as shown in FIG. 3(b), it may be necessary to isolate the pump sources from pump power emitted by the other pump source.

In FIG. 3(c) embodiments, dichroic filters 32 are provided to introduce the pump power from the pump sources 421i and 422i to amplifying fiber 40i. As further shown, the dichroic filters 32 can be used to introduce pump power from additional pump sources 421i−1 and 422i+1 into adjacent amplifier stages 40i−1 and 40i+1, respectively. Likewise, circulators 30 can also be used to provide pump wavelengths in opposing directions as shown in FIG. 3(d).

The embodiments shown in FIGS. 3(e-l) are exemplary of pump source 42 configurations to provide co-directional pump wavelengths λp1 and λp2. As shown in FIG. 3(e), the pump sources 42 can be configured to supply pump power, either locally or remotely, to a plurality of amplifying fibers 40 in one or more amplifiers 12. One or more combiner stages can be used to combine the pump wavelengths λp1 and λp2 as shown in FIG. 3(f). Couplers 28, circulators 30, and dichroic devices 32 can be used in various combinations, as exemplified in FIGS. 3(g-l), to provide co-directional pump power in pump wavelengths λp1 and λp2 to the amplifying fiber 40.

Multiple pump sources 42 can be used to supply power to the amplifier 12 during normal operation. For example, a third pump source 423 can supply power via pumping path 443 to the amplifying fiber as shown in FIG. 3(k). The second pump source 422 can be used to replace power supplied by either or both the first and third pump sources 421 and 423, respectively.

The pump source 42 will generally include one or more fixed or tunable wavelength optical sources 46 to provide power to the amplifying fiber via optical energy carried by one or more pump wavelengths λpi through the pumping paths 44. The optical source 46 will generally include a fixed or tunable wavelength laser, such as DFB, DBR, or other types of semiconductor or fiber lasers. Other coherent or incoherent optical sources can be used as the optical sources 46 depending upon the desired characteristics of the pump source 42.

FIG. 4 shows an embodiment of the pump source 42 including one or more optical source 46N, each providing optical energy, or pump power, in one or more wavelength ranges λpi. The pump wavelengths provided by the optical sources 46N can be stabilized using a reflective element, such as fiber Bragg grating, in a “pigtail” fiber 48 proximate to the source 46N to form an external lasing cavity. Polarization maintaining “PM” fiber can be used as the pigtail fiber 48 to provide additional control over the optical wavelengths emitted by the source 46N. A photodiode 52 and optical source controller 54 can be used to provide feedback control over the optical sources 46N, as is known in the art.

The pump source 42 can also include one or more depolarization devices 50 to minimize polarization dependent amplifier gain variations that can occur in Raman fiber amplifiers and less frequently in erbium doped fiber amplifiers. An example of suitable depolarization devices include two or more serially coupled PM fiber sections in which the polarization planes of consecutive PM fibers are rotationally offset, thereby causing a polarization realignment of the light passing through the PM fibers. For example, a 45° rotational offset can covert a polarized beam into a substantially unpolarized beam after passing thought a sufficent length, i.e., >20 m, of the rotationally offset PM fiber. The polarization of the beam exiting the offset PM fiber section can be controlled by varying the offset angle and length of each offset PM fiber section. The PM fiber design or composition can be varied between the sections to further affect the beam polarization, although dissimilar fibers can excessively increase splice losses.

Alternatively, the depolarization device 50 can include a polarization multiplexer for combining two orthogonal polarized beams into an unpolarized beam. The two beams can be provided by tow different optical sources or by one optical source split into tow beams.

In operation, the amplifier 12 will be supplied power from at least the first pump source 421. In some embodiments, the second pump path 442 may be unused during normal amplifier operation. Whereas, in other embodiments, the second pump source 422 can be used to provide power to the amplifying fiber 40 during normal operation. If the first pump source 421 needs to be taken out of service, the second pump source 422 is provisioned to supply pump power to the amplifying fiber 40 via the second pumping path 442. The pump power supplied by the second pump source 422 is used to replace the pump power supplied by the first pump source 421 and control the amplifier performance charcteristics, such as gain and gain profile, over the signal wavelength range during normal amplifier operation.

The second pump source 422 can be removably or fixedly provisioned to supply pump power to the second pumping path 442. Of course, a removable pump source provides a more flexible design, although various fixed embodiments can provide benefits in practice. For example, the pump sources 42 and the amplifying medium/fiber 40 are often located on physically separate parts, such as line cards, in the amplifier 12 to allow separate replacement of the parts. However, the second pump source 422 can be located on the same as the amplifying medium 40 and controlled to supply power when the first pump source 421 is being replaced or in other limited circumstances. This embodiment would eliminate the need for service personnel to install a removable pump before performing a pump maintenance procedure. Broader use of a fixed second pump source 422 can be made, although replacing or repairing a fixed source can increase the complexity of the procedure. Also, both fixed and removable pump source 422 can be provide for use in the amplifier 12 to provide additional flexibility, if desired.

In one aspect, the second pump source 422 can be used to replace the pump power supplied by the first pump source 421 to the amplifier fiber 40. The first pump source 421 can then be removed from the amplified 12, when the power supplied by source 421 has been reduced to a safe level. The first pump source 421 can be repaired or replaced with a replacement pump source, which will typically be a replacement first pump source. The pump power provided by the second pump source 422 can then be replaced by the replacement pump source and the second pump source 422 can be removed. Alternatively, the amplifier can be designed to operate with the second pump source 422 in operation.

The replacement of pump power from one pump source with pump power from another pump source is generally performed to replace one or more failed or degrade optical source 46 in the pump source 42. However, the present invention can be used to change the amplifier performance characteristics, such as upgrading the optical source used in the pump source or installing new pump source to achieve amplification over a different signal wavelength bandwidth. For example, a replacement pump source for Raman amplifier could provide for amplification over wider bandwidth than the first pump source 421 being replaced. The wider band replacement pump source could be operated at the bandwidth of the first pump source 421, until the additional bandwidth was required. Additionally, the bandwidth of the replacement pump source can be narrowed or shifted to meet the changing capacity demands of the system 10.

The power replacement procedure can be performed either manually by service personnel or under computer control. Pump power replacement via computer can be implemented and controlled either locally or remotely depending upon the optical system embodiment. For example, a service technician at an amplifier site can get instructions provide via a craft or other interface to the network management system 22. Per the instructions, the second pump source 422 can be removably connected to an input port to supply pump power through the second pumping path 442 to the amplifying fiber 40. A network management system 22 processor can then reduce the power supplied by the first pump source 421, while replacing the power using the second pump source 422. When the first pump source 421 power has been reduced to a safe level, the network management system processor can notify the service personnel via the craft interface that the first pump source 421 can be removed. Furthermore, the network management system processor can instruct the service technician to install a replacement pump source, if it is to be installed, and then transition the pump power back to being supplied by the replacement pump source. The network management system 22 can again notify the service technician, when the power supplied by the second pump source 422 has been reduced to a safe level. The service technician can then remove the second pump source 422 and complete the procedure.

The amplifier 12 and the network management system 22 can also be configured to recognize the pump wavelengths associated with the second pump source 422 and the replacement pump source and control the power supplied accordingly. Similarly, when an upgraded pump source is provided in the amplifier 12, the network management system 22 can be used to implement any changes to control scheme and wavelength operation of the amplifier 12.

Thus, the present invention provides for optical systems and amplifiers having improved reliability and availability, as well as flexibility in system configuration design. Those of ordinary skill in the art will further appreciate that numerous modifications and variations that can be made to specific aspects of the present invention without departing from the scope of the present invention. It intended that the foregoing specification and the following claims cover such modifications and variations.

Claims (14)

What is claimed is:

1. An optical system comprising:

at least one optical transmitter configured to transmit an optical signal through an optical transmission fiber;

an optical amplifier disposed along said transmission fiber to amplify the optical signal;

a first removable pump source connected to said amplifier via at least a first pumping path;

at least a second pumping path connected to said amplifier, wherein the second pumping path is unused during normal amplifier operation;

a second pump source connected to said second pumping path, wherein said second pump source is not used during normal amplifier operation;

means for replacing the power supplied by said first pump source to said amplifier with power supplied by said second pump source via said second pumping path when the first pump source is to be replaced;

means for replacing power supplied by said second pump source with power supplied by a replacement for said first pump source after said first pump source has been replaced; and,

at least one optical receiver configured to receive the optical signal from said transmission fiber.

2. The system of claim 1, wherein said first and second pump sources are removable and configured to replace power supplied by the other of said first and second pump sources and control performance characteristics of said amplifier.

3. The system of claim 1, wherein:

said optical amplifier includes a fiber amplifier supplied with optical energy; and,

said first pump source provides optical energy in at least one pump wavelength to said fiber amplifier.

4. The system of claim 3, wherein said first and second pump sources include a plurality of lasers providing optical energy in one or more pump wavelength ranges.

5. The system of claim 3, wherein said first and second pump sources provide optical energy in pump wavelengths within at least one of 1460-1500 nm and 960-990 nm wavelength ranges.

6. The system of claim 3, wherein:

said amplifier is an erbium doped fiber amplifier;

said first pump source includes a plurality of lasers providing optical energy in pump wavelengths ranging from 1460-1500 nm range; and,

said second pump source includes a plurality of lasers providing optical energy in pump wavelengths ranging from 960-985 nm range.

7. The system of claim 3, wherein said fiber amplifier is selected from the group consisting of doped fiber, Raman gain fiber, and combinations thereof.

8. The system of claim 3, wherein said first and second pump source supply power to said fiber amplifier in pump wavelengths propagating toward at least one of said at least one transmitter and said at least one receiver.

9. The system of claim 3, wherein said first and second pump sources provide optical energy in pump wavelengths that stimulate Raman gain in the optical signals.

10. The system of claim 3, wherein said first and second pump sources provide optical energy in pump wavelengths to produce gain in said fiber amplifier.

11. The system of claim 1, wherein:

said optical amplifier includes an erbium doped fiber amplifier;

said first pump source provides optical energy to said amplifier via said first pumping path propagating toward said at least one transmitter; and,

said second pumping path is configured to supply optical energy propagating toward said at least one receiver.

12. The system of claim 1, wherein:

said optical amplifier includes a Raman fiber amplifier;

said first pump source provides optical energy to said amplifier via said first pumping path propagating toward said transmitter; and,

said second pumping path is configured to supply optical energy propagating toward said transmitter.

13. The system of claim 1, wherein said first and second pump sources are operated at different pump wavelengths and powers to control the gain profile of said amplifier.

14. An apparatus comprising:

an amplifying fiber employing optical energy to amplify optical signals in said fiber;

a first removable pump source connected to said amplifying fiber via at least a first pumping path;

at least a second pumping path connected to said amplifier, wherein said second pumping path is unused during normal amplifier operation;

a second pump source connected to said second pumping path, wherein said second pump source is not used during normal amplifier operation;

means for replacing the optical energy supplied by said first pump source to said amplifying fiber with power supplied by said second pump source via said second pumping path when the first pump source is to be replaced;

means for replacing power supplied by said second pump source with power supplied by a replacement for said first pump source after said first pump source has been replaced.